Role of Redox-Inactive Metal Ions in Modulating the Reduction Potential of Uranyl Schiff Base Complexes: Detailed Experimental and Theoretical Studies

Abstract

A mononuclear uranyl complex, [UO₂L] (<b>1</b>), has been synthesized with the ligand <i>N</i>,<i>N</i>'-bis(3-methoxy-2-hydroxybenzylidene)-1,6-diamino-3-azahexane (H₂L). The complex showed a reversible U(VI)/U(V) redox couple in cyclic voltammetric measurements. The reduction potential of this couple showed a positive shift upon the addition of redox-inactive alkali- and alkaline-earth Lewis acidic metal ions (Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, and Ba²⁺) to an acetonitrile solution of complex <b>1</b>. The positive shift of the reduction potential has been explained on the basis of the Lewis acidity and internal electric-field effect of the respective metal ions. The bimetallic complexes [UO₂LLi(NO₃)] (<b>2</b>), [UO₂LNa(BF₄)]₂ (<b>3</b>), [UO₂LK(PF₆)]₂ (<b>4</b>), [(UO₂L)₂Ca]·(ClO₄)₂·CH₃CN (<b>5</b>), [(UO₂L)₂Sr(H₂O)₂]·(ClO₄)₂·CH₃CN (<b>6</b>), and [(UO₂L)₂Ba(ClO₄)]·(ClO₄) (<b>7</b>) have also been isolated in the solid state by reacting complex <b>1</b> with the corresponding metal ions and characterized by single-crystal X-ray diffraction. Density functional theory calculations of the optimized [UO₂LM]^<i>n</i>+ complexes have been used to rationalize the experimental reduction and electric-field potentials imposed by the non-redox-active cations.